Teletext is a transmission system in which alpha-numerical and graphic information is transmitted via some picture lines of a television signal. The information is composed of pages and a plurality of such pages is transmitted cyclically. Each page is identified by a page number. In many countries, including virtually all European countries, teletext services are accessible to and very popular with the general public.
Television receivers including a teletext decoder are generally known. In their simplest form they enable the user to enter a desired page number whereafter the teletext decoder acquires, stores and displays the page having this number. Generally, it will take some time before the desired page is transmitted in the cycle and is received and displayed by the teletext decoder. In order to be able to transmit as many pages as possible in a given cycle, the broadcasting station often resorts to rotating pages. These are teletext pages having the same page number but whose contents rotate. Such pages will hereinafter be referred to as sub-pages. For example, a television program survey is allocated to two sub-pages. During one cycle the page then comprises the afternoon program, during a further cycle it comprises the evening program. If the user enters the relevant page number, the sub-page subsequently received as the first page with one of the two surveys is displayed. The sub-page with the other survey automatically follows one cycle later.
European Patent Application EP-A 0 290 000 discloses a television receiver including a teletext decoder in which a control signal is applied to the decoder for stopping the automatic rotation after the user has depressed a "stop" key. Such a television receiver enables the user to read a displayed sub-page at his leisure without being disturbed by the reception of a subsequent sub-page. However, in this case the user should realise that the actual page is a rotating page. This is often not the case. The editor of the page is at liberty to indicate whether the page is a rotating page. If he indicates this, he will often restrict himself to a symbol "&gt;", which means "next" or "1/2", which means "page 1 of 2". Such indications are often unobtrusively displayed in a corner of the page. If the user overlooks such an indication and consequently does not depress the "stop" key, the automatic rotation will catch him by surprise while he is reading the page.
A further drawback of the known television receiver is that the display of "stopped" sub-pages is actually frozen. Generally this need not have been the user's intention. This will now be illustrated by way of an example. Four sub-pages having page number 570 comprise the stock exchange rates. Such data are continuously refreshed at the transmitter end. The user depressing the "stop" key when sub-page 3 with the exchange rate interesting to him is being displayed will now be deprived of a further refreshing of this sub-page. He should realise that he must "de-stop" the sub-page and wait for another reception of this sub-page.
The teletext decoder in the known television receiver further includes a multipage memory for alleviating other detrimental aspects of rotating pages. More particularly, the decoder is adapted to store in this multipage memory those teletext pages which are associated with the same rotating set and are transmitted after the "stopped" page. After having read the "stopped" sub-page, the user can call the next page with the same page number from the memory. Such a television receiver provides great convenience of use. However, in practice the receiver appears to have some drawbacks. For example, in practice it may occur that a broadcasting station transmits one and the same sub-page in two successive cycli. The reason is that the broadcasting station considers a reading time of one cycle insufficient and extends the reading time to two cycli in this manner. If a user stops the first sub-page (in order to determine its reading time himself) and if he subsequently calls the next page from the memory, this next page may appear to be a duplicate of the sub-page which he has just read. This will confuse the user. Moreover, storage space in the television receiver is wasted in this manner.
Another drawback of the known television receiver is that the available multipage memory will fill up as long as the user reads a "stopped" sub-page, even if the rotating page comprises only a small number of sub-pages.